from sqlalchemy import func
from sqlalchemy.exc import IntegrityError
from sqlalchemy.orm import Session

from .dsl import QuerySpec, build_query
from .eager import default_eager_policy

class CrudService:
    def __init__(self, session: Session, eager_policy=default_eager_policy):
        self.s = session
        self.eager_policy = eager_policy

    def create(self, Model, data, *, before=None, after=None):
        if before: data = before(data) or data
        obj = Model(**data)
        self.s.add(obj)
        self.s.flush()
        if after: after(obj)
        return obj

    def get(self, Model, id, spec: QuerySpec | None = None):
        spec = spec or QuerySpec()
        stmt = build_query(Model, spec, self.eager_policy).where(Model.id == id)
        return self.s.execute(stmt).scalars().first()

    def list(self, Model, spec: QuerySpec):
        stmt = build_query(Model, spec, self.eager_policy)
        count_stmt = stmt.with_only_columns(func.count()).order_by(None)
        total = self.s.execute(count_stmt).scalar_one()
        if spec.page and spec.per_page:
            stmt = stmt.limit(spec.per_page).offset((spec.page - 1) * spec.per_page)
        rows = self.s.execute(stmt).scalars().all()
        return rows, total

    def update(self, obj, data, *, before=None, after=None):
        if obj.is_deleted: raise ValueError("Cannot update a deleted record")
        if before: data = before(obj, data) or data
        for k, v in data.items(): setattr(obj, k, v)
        obj.version += 1
        if after: after(obj)
        return obj

    def soft_delete(self, obj, *, cascade=False, guard=None):
        if guard and not guard(obj): raise ValueError("Delete blocked by guard")
        # optionsl FK hygiene checks go here
        obj.mark_deleted()
        return obj

    def undelete(self, obj):
        obj.deleted = False
        obj.version += 1
        return obj